1. Field of the Invention
This invention relates to improvements in the construction of a ceramic filter making use of bulk waves wherein within a ceramic, an input electrode group and an output electrode group are spaced a predetermined distance apart from each other and overlap each other in the direction of the thickness of the ceramic.
2. Description of the Prior Art
Japanese Patent-Laying Open Gazette No. 85613/1983 discloses an example of such ceramic filter. The ceramic filter disclosed in said Japanese Patent-Laying Open Gazette No. 85613/1983 is shown in a perspective view in FIG. 1. This multi-layered type ceramic filter 1 comprises a piezoelectric ceramic 2 treated for polarization in the direction of the thickness, i.e., in the P direction, and two electrodes groups 3 and 4 disposed within the piezoelectric ceramics 2.
One electrode group 3 forms an input electrodes group, while the other electrodes group 4 forms an output electrodes group. The electrode groups 3 and 4 are composed of a plurality of interdigital electrodes 3a, 3b, 3c, 3d and 4a, 4b, 4c, 4d, respectively, overlapping each other in the direction of the thickness of the piezoelectric ceramic 2. In the input electrodes group 3, the electrodes 3a and 3c are interconnected and brought out together to the periphery of the ceramic. The other electrodes 3b and 3d are likewise interconnected and routed. This manner of lead-out is also applied to the other electrodes group.
When a voltage is applied across the input electrodes group 3, vibrations are transmitted in the direction of the thickness, i.e., in the vertical direction since the electrodes 3a, 3b, 3c and 3d overlap each other in the direction of the thickness. These vibrations are transmitted to the mechanically coupled electrodes group 4. That is, vertical vibrations produced by the input electrodes group 3 are converted into transverse vibrations which are then transmitted. The transmitted vibrations cause a displacement of acoustic waves in the direction of the thickness of the device, i.e., in the vertical direction in the output electrodes group 4, whereby a voltage is produced. The produced voltage is available the electrodes 4a and 4c and the electrodes 4b and 4d of the output electrodes group 4.
With the conventional multi-layered type ceramic filter 1, where vibrations are transmitted from the input electrodes group 3 to the output electrodes group 4, the direction of displacement is changed twice, as described above, and, consequently, the efficiency of the device is very low. Another drawback is that with the transverse displacement, i.e., transverse effect, an unnecessary spurious vibration is produced in the output waveform.
Japanese Patent-Laying Open Gazette No. 85614/1983 discloses a ceramic filter usable in the high frequency region. FIG. 2 show this ceramic filter in a schematic front view. Within the piezoelectric ceramics 5, two electrodes groups 6 and 7 are disposed. The electrodes group 6 forms an input electrodes group, while the electrodes group 7 forms an output electrodes group. The electrodes groups 6 and 7 are composed of a plurality of electrodes 6a . . . 6d and 7a . . . 7d, respectively, overlapping each other, the ceramic layers between adjacent electrodes being polarized in opposite directions as indicated by arrows. In the electrodes groups 6 and 7, input and output terminals are connected to the two electrodes 6a and 6d and the two electrodes 7a and 7d, respectively, positioned on the opposite outer sides close to the piezoelectric ceramic surfaces. Because of such construction, a ceramic filter is attained which is capable of effectively suppressing spurious vibrations due to primary resonance.
With the conventional ceramic filter shown in FIG. 2, however, because vibrations are transmitted from the input electrodes group 6 to the output electrodes group 7, since the direction of displacement is changed twice, and the efficiency remains is very low. Another drawback is that with the transverse displacement, i.e., transverse effect, an unnecessary spurious vibration is produced in the output waveform.